Asthma is the second most common chronic respiratory disorder seen by physicians in outpatients, and has been estimated to be the primary or secondary cause of 9.3 percent of all physician office visits for ambulatory medical care in the United States. Recent reports citing increased asthma morbidity and mortality in this country and abroad have intensified interest in this disorder. Investigations into possible explanations for these increases, however, have been hampered by the lack of good longitudinal data on the incidence, prevalence, and health care utilization associated with asthma, and by the lack of generally accepted diagnostic criteria for defining the disease. This study will provide information on morbidity and health care utilization for asthma for a large, well-defined population over a period of 20 years. Data to be used for this analysis are derived from a unique database consisting of the abstracted medical records of the members of a large, prepaid health maintenance organization (Kaiser Permanente, Northwest Region). This database, which was initiated in 1966, has two main components. The first contains abstracted information on all outpatient contacts for a five percent random sample of the health plan membership, which currently includes over 300,000 members. The second component of this database consists of a similar abstraction system for 100 percent of all inpatient admissions. Use of this database will enable examination of: secular trends in the incidence and prevalence of diagnosed asthma (both inpatient and outpatient) over a period of 20 years; factors that influence these trends; patterns of health care utilization among asthmatics; and the natural history of asthma among selected age subgroups.